In the early 1990s, the U.S. Geological Survey and
several other government agencies funded a little-noticed study of
the effect of a major drought on the Colorado River. Researchers
were particularly interested in its impacts on Lakes Powell and
Mead, which store about four years’ worth of the
river’s average annual flow and serve as the system’s
backup batteries in drought.

The researchers, working
with Boulder, Colo., water-resources engineer Ben Harding, wanted a
worst-case scenario. To find it, they turned to the growing record
of long-term climate data derived from tree-ring studies (HCN,
1/24/05: Written in the Rings). In the tree rings, they found a
drought in the 1500s that lasted for 38 years. Its first years were
the worst, followed by progressive improvement. To turn the drought
into a real beast, Harding flipped it around so that each
subsequent year got worse. Then he built a computer model of the
river based on the Bureau of Reclamation’s own model, and
programmed in the imaginary "severe sustained drought."

In Harding’s drought disaster scenario, the water level in
Lake Powell nose-dived like a Disneyland log-flume ride until, 18
years into the drought, it bottomed out. The reservoir had reached
"dead storage," the point at which no more stored water could be
released downstream out of Glen Canyon Dam. Lake Powell was, for
all intents and purposes, empty — and would stay that way for
eight years.

The drought used in the model was without
real-world precedent. As Harding says, "We’d come up with
this drought that was really perverse. It was just the worst kind
of drought you could possibly contemplate." But what’s
happening on the river now could be even more severe.

A
year ago, curious about how the current drought compared with the
monster he’d created in his computer, Harding dusted off his
10-year-old graph and superimposed the current drought on it.

"I was really surprised," he says. The line traced by the
current drought plunged downward even faster than that of the
simulated drought, thanks not only to hydrologic conditions, but
also to intense water demand in Arizona, California and Nevada.
"The bottom line is that, going into this drought, we’re in
much worse shape, because our reservoirs are lower and our water
uses in the Lower Basin are higher (than was modeled)."

River flows during the past five years have been lower than at any
comparable period in the past century — and clearly show that
there’s far less water in the river than originally thought.
When negotiators signed the Colorado River Compact in 1922, they
believed that the long-term average, measured at Lee’s Ferry,
the dividing line between the Upper and Lower Basins, was 16.2
million acre-feet. That number has proven to be wildly optimistic.
Subsequent reconstructions of the river’s flow over the past
approximately 500 years, extrapolated primarily from tree-ring
data, have shown a long-term average of only 13.5 million
acre-feet. The average annual flow at Lee’s Ferry during the
past five years has been closer to 9.9 million acre-feet.

The Colorado River’s dams and reservoirs have helped buffer
the region against the drought. The Central Arizona Project has
estimated that, without Lake Powell, the Interior Department might
have declared an official shortage on the river in 2002. The
shortage could have become big enough to affect California and
Nevada as well as Arizona by 2003. But even with Lake Powell, an
official shortage could be just a couple of years away.

Could a wet winter break the cycle? Even if this proves to be what
Harding calls "a bang-up year," it hardly means the drought is
over. "If you go back and look at the original drought (in the
1500s)," he says, "it’s got a bunch of wet years in it. But
it just had this relentless aspect to it that kept it dry for a
long, long time."

Harding’s comparison of
the current and severe sustained droughts can be viewed online at
www.hydrosphere.com/publications/SSDRedux.htm